This invention relates to helicopters, and more specifically, to air-driven helicopters in which a flow of air is conveyed through a central hollow rotor shaft through hollow rotors to exit at the tips of each of the rotor blades to provide the necessary rotation of the rotor blades.
The most common construction for helicopters which historically has been used and which generally is employed both in commercial and military helicopters at the present time is to cause the rotor of the helicopter to rotate by mechanical interconnections of the rotor to an engine through various types of mechanical transmissions. Several disadvantages are inherent in such conventional systems, whether a reciprocating internal combustion engine is used as the primary power source, or whether a turbine engine is employed. A primary disadvantage is that significant high torque loads are placed on the structure which is used to support the vertical shaft connecting the rotor blades with the engine. This torque must be counteracted to prevent a counterrotation of the body of the helicopter with respect to the rotor. Typically, this is accomplished by the provision of an additional small tail rotor which is mounted in a vertical plane with its hub at right angles to the helicopter body. The speed of rotation or force which is applied by the tail rotor to the body of the helicopter must be coordinated with the driving power applied to the main rotor blades in order to obtain stable operation of the helicopter. An enormous amount of stress is placed on the body members of the helicopter as well as the transmission or drive chanin used to interconnect the engine with both the main rotor and the tail rotor.
Another significant disadvantage of such conventional helicopter structures, as described above, is in the relatively large number of rotating and moving parts which must be utilized to drive the two rotors. Many bearings, operating under significant stress (rotational, centrifugal, and the like), must be employed. These bearings are costly and require frequent and expensive maintenance.
Not only must the hub design for the rotor be such as to permit rotation about the vertical rotor shaft, but the hub also must be capable of angular movement in vertical planes passing through the vertical rotor shaft. Typically, this angular movement is up to ten or fifteen degrees. To effect such angular control, the rotor blades are tilted through what is known as a swash plate at the hub. The swash plate then is connected to manual controls within the helicopter in a known manner through a plurality of control rods. These control rods are independent of the rotating vertical rotor and are fully self-supporting since the control system cannot be mechanically connected to the rotating rotor.
Conventional mechanically linked rotor drive mechanisms also are sensitive to slight imbalances between the various rotor blades which cause the center of gravity of the rotor blades to be different from the central axis of the rotating rotor. This also places considerable structural loads upon the bearings which support the rotating rotor. In addition, it is difficult to support the rotating rotor very near the rotor hub because of all of the various vearings, support structure, and the like. Generally, the centrifugal force provided by the blades of the rotor is concentrated at a point above the hub which results in further stress on the hub and the rotor support structure.
Because of the large number of moving parts subjected to considerable stress in a conventional helicopter, the maintenance and repair hours considerably exceed the actual flight hours of the helicopter. Consequently, maintenance is a significant cost factor to be considered for the operation of such a conventional helicopter.
To reduce the mechanical and structural problems which are inherent in conventional helicopter systems as described above, other approaches have been attempted in the past. One such approach was to place a jet engine or turbine at the tip of each of the rotor blades. This removed all of the structural requirements placed upon the vertical rotor shaft in conjunction with the interconnection of the rotor with an engine located within the body of the helicopter. The rotor blades then simply were connected to a simple rotating disk having its center at the vertical rotor support shaft. Significant fuel delivery problems were substituted for the simplification of the mechanical drive, however. It became necessary to transport the fuel from the helicopter body through the rotor support shaft and into the rotating rotor blades to the engines. An extreme safety hazzard was present because of the high volatility of the fuel and leaks between the hub at the non-rotating rotor shaft and the rotating rotor blade hub were difficult to prevent. In addition, the centrifugal force acting upon the fuel due to the rotating rotor blades changed depending upon the speed of the blades, resulting in a too rich or too lean fuel mixture supplied to the engine. Consequently, power failures occurred.
To take advantage of the simplified structural requirements with the rotor blade acting as a simple rotating disk but without the problems of conveying volatile fuel to jet engines mounted on the tips of the rotor blades, various designs utilizing the flow of pressurized air delivered through a hollow rotor shaft to hollow blades have been developed. In systems using this type of design, a flow of air passes through the rotor blades to nozzles located at the tips of each of the blades and directed rearwardly. Consequently, air discharging through the nozzles results in reactive force in the opposite direction and rotates the blade about the hub. A variety of attempts to develop practical helicopters utilizing this concept of an air-driven rotor have been made in the past.
One of the more recent air-driven helicopter systems is disclosed in the patent to Nagler, U.S. Pat. No. 3,830,588. This patent discloses an air-driven helicopter which has a hollow rotor shaft connected to an air compressor to convey a flow of air through the rotor shaft to the rotor hub. The hub rotates about the fixed shaft on ball bearings, with the center line of the rotor blades concentrating a center of force which is primarily above the bearings. Separate air seals and a separate spherical bearing to handle the tilt of the rotor blades is required. In addition, flexible bellows are provided to accomodate the tilt functions of the rotors effected by the swash plate. Because of the relatively high temperature of the compressed air which is used in such a helicopter, the ball bearings and the air seals of the system disclosed in this patent require frequent maintenance.
A system which is similar in some respects to the Nagler system is disclosed in the patent to Abramopaulos, number 3,612,441. This patent has the rotar rotating on ball bearings located above the center of force, with a spherical bearing separately utilized to handle the tilt of the rotor assembly. The spherical bearing also, apparently, is used to effect the air seal to prevent the compressed air from leaking from the system during operation. This patent also is subject to the shortcomings of any system using ball bearings in the high temperature environment which results in lubrication problems and frequent maintenance. In addition, because of the arrangement of parts, tight sealing off of the air to prevent any leakage of the air used to drive the rotor is difficult.
Three other patents which are directed to air-driven helicopter rotors and which are typical of approaches taken in the prior art to implement this technique are the patents to Laufer, U.S. Pat. No. 3,073,394, Ryan et al, U.S. Pat. No. 3,159,360, and Pullin, U.S. Pat. No. 2,429,646. All three of the systems disclosed in these patents use ball bearings at the hub to handle the rotation of the hub relative to the rotor shaft fixed to the air frame of the helicopter. In Laufer, a large separate sealing ring is employed along with flexible air pipes to accommodate the rotor tilt. The separate sealing ring and the ball bearings are subject to the disadvantages noted above in conjunction with the Nagler and Abramopaulos systems.
In Ryan, as in Laufer, a large separate sealing ring is employed along with the ball bearings at the hub. The Pullin patent passes the compressed air through a fixed sphere with an annular slot in it aligned with the ends of the rotating rotor blades. Separate gas sealing rings are employed to prevent leakage of the high pressure air from the rotor hub assembly.
In the Laufer, Ryan and Pullin patents, the center of force of the rotating rotor blades is generally aligned with the hub; so that the disadvantages of the location of the center of force above or below the bearings which is present in the Nagler and the Abramopaulos patents is not present in these three systems. All of these systems, however, still are subject to the significant disadvantages encountered in the lubrication of ball bearings in the high temperature regions encountered and in the provision of efficient and long-lasting seals to prevent leakage of the air through the hub/rotor interface.
Other prior art patents for air-driven helicopter rotors, which are subject to the same disadvantages present in the systems described specifically above, are found in the patents to Laufer, U.S. Pat. No. 2,845,131; Andrews, U.S. Pat. No. 3,119,577; Leoni, U.S. Pat. No. 3,370,809; French Pat. No. 1,002,007 (October 1951); and Italian Pat. No. 419,603 (April 1947).
The concept of a compressed air-driven helicopter rotor theoretically appears to be an ideal concept for powering a helicopter. By causing the reactive forces to take place at the tips of the rotor blades, no counterrotating stress is applied from the rotor to the body of the helicopter; so that the additional tail rotor which is conventionally used is not necessary. In addition, a truly free-wheeling rotor may be employed without the necessity of any complex clutch arrangements, or the like; so that in the event of a power failure, the rotor immediately is able to go into an auto rotation mode to permit safe landing of the helicopter. It is desirable, therefore, to provide a hub mechanism or hub system which is not subject to the disadvantages of the prior art, particularly with respect to the bearings and air seals; so that the inherent advantages of an air-driven helicopter rotor may be commercially realized.